1. Field of the Invention
The invention relates to a recording apparatus at least capable of recording with respect to a hologram recording medium and a phase modulation device preferable to be used in the recording apparatus.
2. Description of the Related Art
In a hologram recording/playback system, particularly in a hologram recording/playback system in a field of optical storage, a SLM (spatial light modulator) such as a liquid-crystal panel or DMD (Digital Micromirror Device) is used as light intensity modulation is used, in which intensity modulation whereby a pattern arrangement of a bit 1 (for example, light intensity=high) and a bit 0 (for example, light intensity=low) can be obtained is applied to signal light.
At this time, in the SLM, signal light which received the light intensity modulation according to recorded data is formed at the central part and reference light transmitted so as to make a circle around the signal light is formed as shown in FIG. 2. The signal light modulated according to recorded data is radiated to the hologram recording medium with the reference light, accordingly, interference fringes of signal light and reference light are recorded in the hologram recording medium as data.
At the time of playing back data, only the reference light is generated in the SLM and is radiated to the hologram recording medium to obtain diffracted light according to the interference fringes. Images according to the diffracted light are focused on an image sensor such as a CCD (Charge Coupled Device) sensor or a CMOS (Complementary Oxide Semiconductor) sensor to perform image detection. Playback data is obtained based on image data detected in this manner.
A hologram recording/playback system in which signal light and reference light are radiated on the same optical axis is known as a coaxial system (or a collinear system).
In related arts, when such coaxial system is applied, a phase modulation by a phase mask is further given to a real-image side thereof to which spatial light modulation is applied in the SLM as described in JP-A-2006-107663(Patent Document 1).
The phase modulation by the phase mask is performed with respect to both signal light and reference light. The reason that the phase modulation is performed to the reference light is for enabling multiple recording in the hologram recording medium as described in Patent Document 1. That is, signal light (data) recorded by using reference light including a certain phase structure can be read out only by radiating reference light having the same phase structure at the time of playback. Therefore, applying the above, data is respectively recorded by using reference lights having different phase structures at the time of recording and the multiple-recorded data can be selectively read out by selectively radiating the reference lights having these phase structures at the time of playback.
In addition, the reason that the phase modulation is given to the signal light is for performing interference control and for suppressing a DC component with respect to the signal light in the hologram recording medium to increase the recording density.
The invention described in Patent Document 1 intends to perform the optimum phase modulation to signal light and reference light respectively by allowing a phase structure given to signal light and a phase structure given to reference light to be different patterns respectively.